This invention relates generally to photomask assemblies used in a lithographic process and, more particularly, to photomask assemblies incorporating porous frames, such frames being configured to facilitate purging of the space adjacent to the photomask substrate. This invention also relates to methods for making such photomask assemblies.
In the semiconductor industry, intricate patterns of electronic chips are generally made using photolithographic processes. These processes utilize photomask assemblies, in combination with laser exposure systems, to transfer patterns onto electronic chips. FIG. 1 shows the components of a conventional photomask assembly 10, including a pellicle 12, a frame 14, antireflective films 16, a liquid coating 18, a photomask substrate 20, mounting adhesive 22, a cover adhesive 24, glue 26, and a release liner 28.
The photomask substrate 20 typically is made of synthetic silica and is printed with a pattern of an electronic circuit or chip (not shown in FIG. 1) to be produced. The pellicle 12 typically is made of a soft transparent polymer, and it functions to protect the patterned surface of the photomask substrate from outside contaminants, thereby extending the substrate's lifetime and decreasing the production costs of the electronic chips. Both surfaces of the pellicle are coated with antireflective films 16, to increase the transmittance of the laser light. The frame 14 typically is made of anodized aluminum, and it functions to support the polymer pellicle above the substrate so as to define a pellicle space 32. The frame typically has a rectangular shape, although it alternatively can have a polygonal, oval, or circular shape. The frame and pellicle typically are mounted on the photomask substrate using any of a variety of adhesives. The release liner 28 typically is made of a polymer material, and it functions to facilitate an easy removal of the pellicle, to allow various components of the assembly to be cleaned or replaced. The liquid coating 18 is applied to the inner surface of the frame, to capture particulate matter present in the pellicle space 32.
The pattern on the photomask substrate 20 is repetitively transferred onto the surface of a succession of electronic chips (not shown in FIG. 1) by continuously exposing the photomask substrate to light of a specified wavelength. Conventional photolithography systems use laser light sources operating at wavelengths of 436 nm, 365nm, 248 nm, and more recently 193 nm. In general, lower wavelengths provide a finer pattern resolution. It is expected that 157 nm lasers will be used in the future to develop even finer patterns.
The exposure to light from high energy lasers operating at such ultraviolet (UV) and deep-ultraviolet (DUV) wavelengths can heat the assembly and trigger certain undesired photochemical and thermal reactions. These reactions can cause defects to form and grow on the surfaces of the components of the photomask assembly, eventually destroying the patterns transferred to the chips.
The formation and growth mechanisms resulting from the undesired photochemical and thermal reactions identified above are described in a publication by Bhattacharyya et al., entitled “Investigation of Reticle Defect Formation at DUV Lithography,” BACUS News, February 2003, Vol. 19 (2). These mechanisms are affected by several factors, including the photomask assembly components described above, the assembly container, the storage and fabrication environment, the exposure system environment, residuals from the cleaning of the assembly components, repetitive exposure to the laser light, and the wavelength of the laser. The Bhattacharyya et al. publication reports that outgassing of ammonia from the frame adhesive is responsible for forming defects at 248 nm exposure. It also reports that the number of defects increases considerably after the 700th exposure to a 193 nm laser. These defects might form due to the presence of water vapor, ammonia, carbon dioxide, and sulfuric acid, which either have diffused into the pellicle space from the outside environment and/or have been formed by degassing or degradation of the assembly components. It is believed that oxygen present in the exposure environment also can cause defects to be formed. In addition, oxygen and water vapor can absorb the laser light at 193 nm, thereby decreasing the light transmittance. Volatile hydrocarbons also can contribute to the formation of defects and to a decrease in transmittance, by absorbing the laser light. Such problems are expected to increase with the future use of higher energy, 157 nm photolithography systems.
The formation of defects can be partially or completely avoided by purging the pellicle space with an inert gas such as nitrogen after the assembly has been fabricated and/or during the laser exposure. This purging removes the harmful chemicals mentioned above. As explained in a publication by Cullins, entitled “LITJ360—157 nm Mask Materials,” International SEMATECH's 157 nm Technical Data Review, December 2001, some incidental purging may occur through the pellicle itself, because the pellicle is formed of a polymer material having some permeability. However, this purging is thought to be too slow to eliminate all the problems discussed above within a reasonable processing time. Also, it is known that the soft polymer pellicles can easily degrade when repetitively exposed to light from UV and DUV lasers, causing considerable reduction in light transmittance, particularly at 157 nm. In addition, soft polymer pellicles cannot easily be cleaned and handled. U.S. Pat. No. 6,524,754 to Eynon suggests that hard pellicles formed of synthetic or fused silica can be substituted for the soft polymer pellicles. Although such hard pellicles can solve the cleaning, handling, and degradation problems, they are impermeable to gases and thereby not suitable for purging through the pellicle.
Conventional photomask assemblies incorporate frames made of anodized aluminum, which have the following significant disadvantages. First, because the aluminum frame is generally impermeable to most gases, the pellicle space cannot be purged through the frame.
Second, there is a considerable mismatch between the coefficient of thermal expansion (CTE) of the aluminum frame and that of the photomask substrate. The high purity synthetic silica widely used in manufacturing of photomask substrates for conventional lithography has a CTE of about 0.55 ppm/° C., which is significantly lower than that of the aluminum frame, about 25 ppm/° C. Fluorinated synthetic silica is considered to be a material of choice for manufacturing of photomask substrates for DUV, particularly for 157 nm lithography. The CTE of the fluorinated silica is affected by the level of fluorine doping. For example, silica articles doped with about 8,000 ppm and about 15,000 ppm fluorine have CTEs of about 0.51 ppm/° C. and about 0.43 ppm/° C. respectively. The hard pellicles also made of silica or fluorinated silica have CTEs similar to those of the photomask substrate. The generation of heat during the manufacturing of the electronic chips causes the temperature of the photomask assembly to increase above the room temperature. Because of the large CTE mismatch between the frame and the substrate, this heating generates significant stresses in the assembly. As a result, the printed area can be distorted, degrading the image transferred on the chip. Also, the pellicle can bend unacceptably, further aggravating the image degradation. These problems become more acute when DUV photolithography wavelengths, such as 193 nm and 157 nm, are used. As reported by Kikugawa et al., in a publication entitled “Current Status of Hard Pellicle Development,” International SEMATECH's 157 nm Technical Data Review (December 2001), the bending of the hard pellicle was intolerable, around 50 micrometers, when the assembly is heated only from 21.6° C. to 26° C.
Third, aluminum frames cannot be machined to better than about a 20-micrometer surface flatness. The sharpness of the image transferred onto the chip highly depends on the optical alignment of the pellicle film with respect to the substrate. Any misalignment causes optical distortions resulting in patterns with bad quality. This is especially a very acute problem for 157-nm lithography. It is therefore preferable to have a frame which is made of a material suitable for grinding and polishing to obtain surface flatness levels better than 20 micrometers.
Another problem encountered by conventional photomask assemblies results from pressure gradients that can arise between the pellicle space and the assembly's exterior, e.g., during shipment of the assemblies using aircraft, exposing the assemblies to varying pressures. U.S. Pat. No. 4,833,051 to Imamura, U.S. Pat. No. 5,529,819 to Campi, U.S. Pat. No. 5,344,677 to Hong, and U.S. Pat. No. 5,814,381 to Kuo disclose photomask assemblies incorporating vent structures for overcoming this problem by equalizing pressure between the pellicle space and the assembly's exterior. These vent structures, or channels, are constructed by forming channels having sizes in the range of 50 micrometers to 2,000 micrometers. These channels penetrate through the frame and/or the adhesive layers used in mounting the frame to the photomask assembly. The channels cannot by themselves prevent the diffusion of particles smaller than 10 micrometers into the pellicle space from the assembly's exterior, so the channels take the form of tortuous, zigzag-shaped structures, to trap the particles. The installing of filter systems in the channels and/or applying adhesive coatings onto walls of the channels also are disclosed for preventing the diffusion of particles.
The frames disclosed in the Campi, Hong, and Kuo patents are made of aluminum, stainless steel, or like which have higher CTEs than that of the photomask substrate and/or the hard pellicle and, therefore, cannot prevent the image degradation problems described above. Furthermore, the use of tortuous vent structures, filters, and/or adhesives, mentioned above, are considered to unduly complicate the construction of photomask assemblies.
U.S. Pat. No. 6,593,034 to Shirasaki describes an aluminum, stainless steel, or polyethylene frame having a vent structure for use in purging the pellicle space with nitrogen to prevent the absorption of the laser light by oxygen. The vent structure has 500-micrometer holes penetrating through the frame body, so it incorporates a filter system to prevent diffusion of particles smaller than 10 micrometers. The frames described in Shirasaki patent have CTEs higher than that of the photomask substrate and/or the hard pellicle, and it, therefore, cannot prevent the image degradation problems described above. In addition, the need for a vent structure in the frame body and the need for a filter system are considered to unduly complicate the construction of photomask assemblies. Furthermore, frames made of metals such as aluminum, stainless steel, or the like, generally are not suitable for cleaning by aggressive cleaning agents such as acids to remove contaminants, without producing corrosion-inducing metallic contaminants.
A frame suitable for purging photomask assemblies incorporating hard pellicles is described in a publication by Cullins entitled “LITJ360—157 nm Mask Materials,” International SEMATECH's 157 nm Technical Data Review (December 2001). The disclosed frame includes six holes covered with Gortex® filters. The material of the frame and the CTE of the frame, and the sizes of the holes, are not disclosed. The need for filters is considered to unduly complicate manufacturing of the assembly. In addition, the presence of the filters can cause contamination due to outgassing both from the filters themselves and also from adhesives used for mounting the filters.
S. Kikugawa et al. report on the preparation of synthetic silica frames in “Current Status of Hard Pellicle Development,” International SEMATECH's 157 nm Technical Data Review (December 2001). The publication shows that silica frames can be machined to a surface flatness level of less than 1 micrometer. It also shows that a hard pellicle mounted on a synthetic silica frame will bend by only 0.8 micrometer when the assembly is heated from 21.6 ° C. to 26° C. In contrast, a hard pellicle mounted on an anodized aluminum frame will bend by as much as 50 micrometers when heated over such a temperature range. This result indicates that the CTE of the synthetic silica frame closely matches that of the photomask substrate and the hard pellicle. This silica frame has multiple holes, which are reported to be 1,200 micrometers in diameter, to allow purging the pellicle space with nitrogen. Since a single particle smaller than 10 micrometers in size entering the pellicle can render the entire produced photomask useless, this multi-hole fused silica frames are covered with filters to prevent particle contamination. However, the need for filters complicates manufacturing of the assembly and can cause contamination due to outgassing from the filters and also from adhesives used for mounting the filters. Furthermore, since the inner surface area of such frames is very low, the frame's scavenging capability of the volatile contaminants is considered to be only minimal.
It should be apparent that there exists a need for a photomask assembly incorporating a frame made of a porous material having sufficient permeability to allow for acceptable purging rates, pore sizes smaller than 10 micrometers to prevent particles from entering the pellicle space, and sufficient capability to scavenge contaminants, while possessing a CTE compatible with the photomask substrate and/or the pellicle. The present invention fulfills these needs and provides further related advantages.